Cathode-ray tube (CRT) displays and liquid crystal displays are widely used to display text, images, videos, and so on. Although such display devices can quickly display digital data and rewrite displayed images, they are not suitable to carry around. Also, since such display devices are light-emitting devices, viewing displayed images for a long time strains the eyes. Further, those display devices cannot keep images displayed when turned off.
Meanwhile, to distribute or archive text and images as documents, they are printed on paper. Text and images printed on paper are called hard copies. Hard copies reflect multiply-scattered light and therefore have better visibility and are easier on the eyes compared with light-emitting devices. Also, hard copies are lightweight and easy to handle, and therefore can be read in any posture. However, hard copies become waste after use. Although a part of used hard copies can be recycled, recycling is labor-intensive and costly. Thus, hard copies have a disadvantage in terms of resource savings.
With the advancement of information technology, more and more information is being processed using computers, and we have more chances to read documents on a display. Against this backdrop, there is an increasing need for a paper-like display that has advantages of both an electronic display and a hard copy, i.e., that is rewritable and suitable for reading documents. As materials for a bright, reflective paper-like display with a memory capability, polymer-dispersed liquid crystals, bistable cholesteric liquid crystals, electrochromic elements, and electrophoretic display elements are drawing attention. Especially, a display using electrophoretic display elements has an advantage in terms of display quality and power consumption during operation.
In an electrophoretic display element, an electrophoretic display medium containing a dispersion liquid, which includes electrophoretic particles dispersed in a dispersion medium having a color different from that of the electrophoretic particles, is placed between a pair of transparent electrodes. The surfaces of the electrophoretic particles in the dispersion medium are electrically charged. When a voltage attracting the electric charge of the electrophoretic particles is applied to one of the transparent electrodes, the electrophoretic particles are attracted to and accumulate at the one of the transparent electrodes and, as a result, the color of the electrophoretic particles becomes observable. When a voltage repelling the electric charge of the electrophoretic particles is applied, the electrophoretic particles move to the other one of the transparent electrodes and, as a result, the color of the dispersion medium becomes observable. An electrophoretic display element displays a color using this mechanism.
To implement an image display device using electrophoretic display elements as described above, it is necessary to arrange a large number of electrophoretic display elements in a small area. For this purpose, a structure for arranging the electrophoretic display elements is necessary. For example, a honeycomb structure sheet, which is an aggregate of hollow structures, is suitable as an image display element structure used to arrange minute electrophoretic display elements. To produce an image display device using a honeycomb structure sheet, electrophoretic display elements each including electrophoretic particles and a dispersion medium are formed in the respective cells of the honeycomb structure.
Patent document 1 discloses an electrophoretic display and a method of producing the electrophoretic display. The disclosed electrophoretic display is produced by filling cup-like recesses, which are formed by micro-embossing or image exposure, with a dispersion of charged pigment particles dispersed in a solvent or a solvent blend; and by curing an overcoat layer over the dispersion to seal the dispersion in the recesses. The overcoat layer is made of a sealing composition that has a specific gravity smaller than that of the dispersion and is at least partly immiscible with the dispersion.
Patent documents 2 and 4 disclose a display using an electrophoretic liquid where three types of particles are dispersed for color display. Patent document 3 discloses a display using an electrophoretic liquid comprising electrophoretic particles and non-electrophoretic particles dispersed in a colored dispersion medium.
[Patent document 1] Japanese Patent No. 3680996
[Patent document 2] Japanese Patent Application Publication No. 2002-511607
[Patent document 3] Japanese Patent Application Publication No. 2001-188269
[Patent document 4] Japanese Patent Application Publication No. 2002-520655
As described above, various electrophoretic display methods and devices are proposed. Also, research is being conducted to develop color electrophoretic display methods and devices. In patent documents 2 and 4, a dispersion of three types of particles dispersed in a dispersion medium or a dispersion of two types of particles dispersed in a colored dispersion medium is used to implement a color display. However, although patent documents 2 and 4 disclose operations of displays, they do not disclose concrete examples of dispersed particles (their characteristics, materials, and production methods). Therefore, with the technologies disclosed in patent documents 2 and 4, it is difficult to produce a color electrophoretic display device.